Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 11 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Investments In Real Estate [Line Items]
Total Investment in Real Estate	$ 970,964	$ 782,166	$ 783,233	$ 595,959
Accumulated Depreciation And Amortization Description [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	914,488
Cost Capitalized Subsequent to Initial Investment	56,476
Land	589,497
Building and Improvements	381,467
Total Investment in Real Estate	970,964
Accumulated Depreciation and Amortization	133,353
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Woburn One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	350
Cost Capitalized Subsequent to Initial Investment	64
Land	200
Building and Improvements	214
Total Investment in Real Estate	414
Accumulated Depreciation and Amortization	$ 202
Date of Initial Leasehold or Acquisition Investment	1986
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Woburn Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 508
Cost Capitalized Subsequent to Initial Investment	394
Land	508
Building and Improvements	394
Total Investment in Real Estate	902
Accumulated Depreciation and Amortization	$ 271
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Worcester Seven [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 400
Land	400
Total Investment in Real Estate	$ 400
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Worcester One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 500
Land	500
Total Investment in Real Estate	$ 500
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Worcester Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 550
Land	550
Total Investment in Real Estate	$ 550
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Worcester Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 548
Cost Capitalized Subsequent to Initial Investment	10
Land	356
Building and Improvements	202
Total Investment in Real Estate	558
Accumulated Depreciation and Amortization	$ 122
Date of Initial Leasehold or Acquisition Investment	1991
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Worcester Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 497
Cost Capitalized Subsequent to Initial Investment	326
Land	322
Building and Improvements	501
Total Investment in Real Estate	823
Accumulated Depreciation and Amortization	$ 294
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Worcester Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 978
Cost Capitalized Subsequent to Initial Investment	8
Land	636
Building and Improvements	350
Total Investment in Real Estate	986
Accumulated Depreciation and Amortization	$ 209
Date of Initial Leasehold or Acquisition Investment	1991
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Accokeek One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 692
Land	692
Total Investment in Real Estate	$ 692
Date of Initial Leasehold or Acquisition Investment	2010
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Baltimore One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 802
Building and Improvements	802
Total Investment in Real Estate	802
Accumulated Depreciation and Amortization	$ 432
Date of Initial Leasehold or Acquisition Investment	2007
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Baltimore Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,259
Land	722
Building and Improvements	1,537
Total Investment in Real Estate	2,259
Accumulated Depreciation and Amortization	$ 759
Date of Initial Leasehold or Acquisition Investment	2007